High frequency oscillators



Filed June 50, 1949 INVENTOR ES TERM F/ELD fi/ M A TTORNEY United States Patent fiice 2,724,775 Patented Nov. 22, 1955 2,724,775 HIGH FREQUENCY OSCILLATORS Application June 30, 1949, Serial No. 102,318

7 Claims. (Cl. 250-36) This invention relates to improvements in high frequency oscillators, and one of its principal objects is to provide oscillators for operation at microwave frequencies which; are continuously tunable throughout a substantial band.

.It is also an object of the invention to provide a tunable microwave oscillator using a travelling wave tube, wherein amplification is effected by the interchange of energy between a travelling wave and an electron stream.

Another object of the present invention is to provide oscillators which will produce useful output at frequencies beyond the range wherein the total net gain of the tube is greater than unity.

The invention will be described with reference to the accompanying drawing, wherein:

Fig. 1 is an elevation, partly in section, embodying the invention,

Fig. 2 is a cross sectional view of a portion of the structure of Fig. 1, taken in the plane 22 of Fig. 1, and

Fig. 3 is a sectional view of a modification of a portion of Fig. 1, using separate feedback and output coupling means. i

Travelling wave tubes can provide amplification throughout very broad bands, of the order of two to one in frequency. These tubes also tend to oscillate, owing to internal feedback. However, in the present state of the art it is not possible to control the internal feedback insuch manner as to vary the frequency of oscillation continuously throughout a wide band. Also, it is difiicult to construct such tubes to provide overall net gain greater than unity at frequencies of the order of. 10,000 megacycles per second and higher. p

According to the present invention, the output end of a travelling wave tube is. coupled to the input end through an external feedback path including a band pass filter resonant at a frequency which is a simple free;

of a system tion (for example, one half) of the" desired output frequency, and harmonics are generated within the tube to provide output energy of the desired frequency.

Referring to Fig. l, the illustrated travelling wave tube includes ahelical conductor 1, an electron gun 3, and a collector electrode 5. The helix 1 is designed so that wave energy applied to it at one end will travel toward the other end with a velocity whichis relatively low, for example, about one-twentieth the velocity of light. The electron gun .3 produces'a beam of electrons. directed along the axis ofthe helix 1 toward the collector electrode 5. The pitch of the helix 1 may be increased near the ends, terminating in straight conductors 8 and 10 which are connected to conductive sleeves 7 and 9 near the input and output ends respectively of the tube.

. The gun 3 includes an accelerator electrode 11. which is maintained at a positive potential withrespect to the cathode 13 by connection to a suitable D. C. source,

not shown The sleeve 7 may be connected jwithin. the accelerator '11. The'accelerating ps tential may be adjusted to make the velocity of the electron stream substantially equal to the velocity of wave propagation on the helix.

The collector 5 may be maintained at the same potential as the accelerator 11, or may be held at a lower voltage, positive with respect to the cathode 13. An evacuated glass or quartz envelope 15 surrounds and supports the tube elements. Magnetic focussing of the electron beam may be provided by an electromagnet 17 surrounding the tube and energized from a D. C. source, not shown. Internal feedback through the tube may be minimized or prevented by a coating of resistive material in the central portion of the envelope 15, as in accordance with known practice.

The external feedback path includes a wave guide 19 extending from the output end of the helix 1 to the input end. The end portions 21 and 23 of the guide 19 are disposed in parallel juxtaposition, with aligned openings 25 provided in their walls to accommodate the tubular envelope 15 of the travelling wave tube. The linear portions 8 and 10 of the helix conductor are positioned within the respective wave guide end portions 21 and 23, and act as probes or antennas for coupling the wave guide to the helix.

The feedback path includes a band pass filter which in the present example comprises two conductive walls 27 and 29 within and extending transversely of the wave guide 19. The walls 27 and 29 include apertures 33 which may be disposed centrally of the respective walls, and are provided with flanges or fingers 31 engaging the inside surfaces of the wave guide 19. The wall 27 is slidable longitudinally of the wave guide 19 and is provided with a lug or tab 35 extending through a longitudinal slot 37 in the wave guide 159 and connected to a threaded rod or leadscrew 39. The leadscrew 39 passes through a stationary nut 41 and may be adjusted by rotation of a knob 43.

A phase shifter is also incorporated in the feedback path for adjusting its effective length. This device may comprise a telescoping wave guide section or trombone, or other equivalent known means. The presently preferred and illustrated phase shifter comprises a relatively long thin body 45 of dielectric material suchas polystyrene, extending longitudinally within the guide 19 and movable laterally thereof between a position of low electric field intensity and a position of high electric field intensity. v In a rectangular guide of the type shown, operating in its simplest and usual mode, the highest electric field intensity occurs at the center of the guide and the lowest intensity is at the side in thevicinity of either of the.

An output wave guide 53 is connected to. the guide at any convenient point between the output endof the.

- travelling wave tube and the filter 27,29. Alternatively,

a probe extending into the guide 19' and coupled to a co' axial transmission line may be used for takingoutpu't from the system. However, it is preferred at present to use the wave guide 53, which may be made of smaller cross sectional dimensions than the guide 19 so as to reject energy of the fundamental frequency and pass only energy of harmonic frequency.

In the operation of the described system, the travelling. wave tube itself functions principally as an amplifier, since internal feedback is prevented as mentioned above. As}; suming second harmonic operation, the filter 27, 29is through one of the openings 33 and out through the other.

At the resonant frequency, energy flowing through the cavity in this manner may be attenuated about one decibel. At other frequencies, the attenuation is consider ably higher. It has beenjfound that if the openings 33 are made large enough to provide the desired low attenuation at resonance, the Q of the cavity will be of the order of 100.

' The phase shifter is adjusted to make the fundamental component of the output of the tube arrive at the input end in proper phase to reinforce the output. There may be several positions of the phase shifter which will pro duce this result, and if so any such position will sutiice. Also, for any given position of the phase shifter, there will be a number of frequencies at which the phase shifts differ by integral multiples of Z-zr, and at which oscillation could occur. However, these frequencies diifer sufficiently so that the relatively low-Q filter 27, 29, substantially rejects all except the one desired.

The accelerating potential may be adjusted initially to make the tube operate simply as an amplifier at the fundamental frequency. Since substantially all of the fundamental output is returned to the input end of the tube, the system will oscillate strongly and the tube Will be driven beyond the linear portion of its input-output characteristic, thus generating harmonics. It has been found that this effect may be made a maximum by careful adjustment of the accelerating potential to a value which generally differs by a few percent from that for maximum fundamental gain. Also, the magnetic focussing field intensity has a much greater effect upon the harmonic generation than upon the fundamental, and should be adjusted critically for optimum result.

Since the travelling wave tube is operative as an amplifier throughout a relatively wide frequency band, the fundamental operating frequency may be selected at any point in such a band by adjustment of the filter and phase shifter. A system like that described has been operated over the band from 7,500 to 11,750 megacycles per second. Notwithstanding the fact that the tube which was used could not provide unity gain much beyond the upper limit of this band, appreciable output was obtained throughout the second harmonic band from 15,000 to 23,500 megacycles per second.

Although a specific embodiment of the invention has been described, it will be apparent that many modifications may be made. For example, other known types of phase shifters and/ or filters may be substituted for those shown. Other known types of travelling wave tubes may be used instead of the particular one described herein. While separate controls for tuning and phase shift are used in the present system, they may be ganged by suitable mechanical connections to operate from a single knob. The output wave guide 53, instead of being connected to the guide 19, may be coupled directly to the helix, either at the lineal portion or at another suitable point.

Referring to Fig. 3, the lineal portion 10 of the helix 1 does not go directly to the sleeve 9, but extends through a region 1' of increased pitch to a further lineal portion a 10' which goes to the sleeve 9. The length of the portion 10 is made such as to provide a good impedance match at the fundamental frequencies to the feedback waveguide 19. The portion 10 is designed for matching the output wave guide 53 at the harmonic frequencies, and is coupled thereto in the same manner as the part 10 is coupled to the guide 19.

Particular emphasis has been placed upon the production of harmonic frequency output by the described oscillator system. However, fundamental frequency energy is also available, and some of it may be extracted if desired by suitable output coupling means such as an additional output wave guide or probe, or by designing the output guide to pass fundamental as well as harmonic frequencies. If substantially pure fundamental output is required, without harmonics, it may be taken from the wave guide 19 between the filter 27, 29 and the input end of the tube, for example by a probe 55 and a coaxial line 57 as shown in Fig. 1.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A microwave oscillator comprising a travelling wave tube capable of providing overall amplification at a frequency 1 but not'at a frequency 2), a feedback path coupling the output end of said tube to the input end of said tube and including an adjustable phase shifter and a band pass filter comprising resonant transmission means tuned to the frequency f, and means for extracting microwave energy from said tube, said last means being designed to pass energy of a frequency which is a harmonic of f.

2. A microwave oscillator comprising a travelling wave tube capable of providing overall amplification at a frequency j", a feedback path coupling the output end of said tube to the input end of said tube and including an adjustable phase shifter and a band pass filter comprising resonant transmission means tuned to the frequency f, and means for extracting microwave energy from said tube, said last means being designed to pass energy of a frequency which is a harmonic of f.

3. A microwave oscillator for providing microwave energy at a harmonic of a selectedfrequency f comprising a travelling wave tube of the type which includes a slow wave propagating structure and means for producing and directing a stream of electrons along said structure, a wave guide coupled at one end to one end of said structure and at the other end to the other end of said structure, said wave guide constituting a feedback path, a phase shifter in said wave guide, said phase shifter being adjustable to make the feedback through said path regenerative at a selected frequency f, means to minimize feedback at frequencies other than 1, comprising a band pass filter in said wave guide including a tunable resonant chamber provided with openings for the passage of high frequency energy of resonant frequency therethrough, means for adjusting said chamber to resonate at said frequency f, attenuator means in the vicinity of the central portion of said wave propagating structure, said attenuator means comprising a body of resistance material and serving to minimize internal feedback in said tube at said frequency f, and means coupled to said structure for extracting energy therefrom, said last means including a wave guide designed to pass energy whose frequency is a harmonic of f but to reject energy of frequency f.

4. A microwave oscillator tunable continuously throughout a relatively wide band of microwave frequencies from f to f for providing microwave output energy over a band of frequencies 2] to 2f, comprising a travelling wave tube of the type which includes a slow wave propagating structure and means for producing and directing a stream of electrons along said structure, an external feedback path including means for extracting energy from the output end of said tube and supplying said energy to the input end of said tube, an adjustable phase shifter in said feedback path, band pass filter means in said feedback path, said filter means being tunable throughout said band of frequencies f to f, and further means for extracting energy from said tube, said last mentioned means including a wave guide device which is capable of passing frequencies 2 to 2 with relatively slight attenuation but ofiers substantial attenuation at frequencies f to f.

5. A microwave oscillator as set forth in claim 4, wherein said Wave guide device is coupled to said external feedback path at a region therealong between said output end of said tube and the phase shifter and filter means.

6. A microwave oscillator as set forth in claim 4, wherein said slow wave propagating structure comprises a helix having first, second and third linear portions spaced from said means for producing and directing said stream of electrons, said external feedback path being coupled to said helix at said first and second linear portions thereof, said Wave guide device being coupled to said helix at said third linear portion thereof.

7. A microwave oscillator tunable continuously throughout a relatively wide band of microwave frequencies from f to f for providing microwave output energy over a band of frequencies 27'' to 2f, comprising a travelling wave tube of the type which includes a slow wave propagating structure in the form of a helix and means for producing and directin" a. stream of electrons along said helix, means for minimizing internal feedback in said tube comprising a band of resistive material surrounding the central portion of said helix, an external feedback path including means for extracting energy from the output end of said tube and supplyingsaid energy to the input end of said tube, adjustable phase shifter means in said feedback path, band pass filter means in said feedback path, said filter means being tunable throughout said band of frequencies f to f, and having a bandwidth corresponding to a phase shift less than 2 radians through said path, and further means for extracting energy from said tube, said last-mentioned means including a device such as a wave guide which is capable of passing frequencies 2 to 2f with relatively slight attenuation but offers substantial attenuation at frequencies f to f.

References Cited in the file of this patent UNITED STATES PATENTS 2,391,386 Bradley Dec. 25, 1945 2,396,044 Pox Mar. 5, 1946 2,404,541 Sloan July 23, 1946 2,408,420 Ginzton Oct. 1, 1946 2,424,959 Alford Aug. 5, 1947 2,427,098 Keizer Sept. 9, 1947 2,458,000 Ingram Jan. 4, 1949 2,512,980 Sunstein June 27, 1950 2,521,760 Starr Sept. 12, 1950 2,580,007 Dohler Dec. 25, 2,607,031 Denis Aug. 12, 1952 FOREIGN PATENTS 591,369 Great Britain Aug. 15, 1947 

